The information age has increased the number of users using data communication systems. Initially, voice was the primary signal carried by phone lines. Next, facsimile (i.e., fax) machines became a popular means for transferring information, though typically restricted to business environments. Recently, with the advent of the Internet, data communications between and among electronic devices has become a common mode of communications for both businesses and individuals. The increase in mode, user group, and usage has driven the telecommunications industry in general, and service providers in particular, to expand capacity. One of the limiting factors of service capacity is the size of detectors for determining dialed digits by various dialing protocols, such as DTMF.
Detection of the dialed digits at the beginning of a phone connection is as old as the PSTN (Public Switched Telephone Network) itself. The requirements are well defined and understood. A current interest is in maximizing the density of DTMF/MF-R1/MF-R2 detection and maintain a high degree of reliability.
In DTMF (Dual-Tone Multi-Frequency) detection, the dialed digits correspond to a row-frequency and a column-frequency, as shown in Table 1. In MF-R1 (Multi-Frequency, One Row) and MF-R2 (Multi-Frequency, Two Rows) detection, two valid frequencies correspond to a dialed digit, although there are no row or column frequencies as in DTMF. Also note that, in MF-R2, two sets of the frequencies are used depending upon the signaling direction. The MF-R1 and MF-R2 frequencies are respectively shown in Tables 2 and 3.
TABLE 1Hz1209133614471633697123A770456B852789C94100#D
TABLE 2Hz7009001100130015001700
TABLE 3FWD138015001620174018601980BWD54066078090010201140
In each case, a bandwidth-test and a twist-test must be passed. In the bandwidth-test, frequency deviations greater than 3.5% must be rejected, and frequency deviations less than 1.5% must be declared as valid digits. In the twist-test, powers at the frequencies of interest must be within certain limits with respect to each other. Furthermore, signals from the line that have power levels less than −40 dBm0 must be rejected. A condition that is particular to DTMF is falsely detecting digits when there is speech activity on the line. This condition is known as talk-off.
One way to increase the density of detectors to allow service providers to support more users is to change (i.e., reduce) the sampling rate of the incoming analog signal. Changing the sampling rate of the incoming signal for detection is not a new idea, and there are numerous patents on the subject. However, this concept is poorly applied in many cases and there is a great loss of efficiency. For example, the sampling rate of conversion is often implemented through finite impulse response (FIR) multi-rate filters (MRF), which are inefficient in terms of memory and complexity. These are usually implemented by filtering followed by a switch for straight decimation operations. Therefore, the filtering is performed at the higher sampling frequency. A better scheme would be the polyphase implementation of an FIR MRF filter. See P. P. Vaidyanathan, Multi-Rate Systems and Filter Banks, Prentice-Hall, 1993. In this representation, the switching comes first and the FIR coefficients are distributed after the switch. In this way, the filtering is performed at the lower sampling rate.